1. Field of Invention
The present invention relates to an image display apparatus using a liquid crystal panel or the like and, more particularly, to an image display apparatus capable of reducing deterioration in image quality due to a non-uniformity of elements while using phase-expanded pixel signals. The present invention also relates to an image display apparatus in which, if an input signal is a digital signal, polarity inversion and phase expansion of the digital signal are executed and digital-to-analog conversion or the like is performed at a low rate. Further, the present invention relates to an image display apparatus which is capable of executing phase expansion a certain number of times at the stages a digital signal and analog signal processed after the digital signal.
2. Description of Related Art
An image display apparatus is known which uses a liquid crystal display panel in which a data-side drive circuit and a scanning-side drive circuit are constituted of thin film transistors (TFTs). In this image display apparatus, matching is necessary between a frequency of a video signal and an operating speed when the video signal is sampled.
Then, a process is conceivable in which pixel signals contained as serial data in the video signal are phase-expanded and a display is made by using the phase-expanded pixel signals. That is, as shown in FIGS. 22A and 22B, a phase expansion circuit 30 for expanding a video signal VIDEO in six phases is provided in a block 10 of a pixel display apparatus. Panel drive video signals V(i) (i=1 to 6) in these phases are respectively output from six output terminals OUT1 to OUT6 in accordance with a control signal from a timing circuit block 20. Panel drive video signals V(i) are supplied to groups of data signal lines 112 of a liquid crystal panel 110 each corresponding to a row of six pixels in the horizontal direction via sampling switches 134 connected to signal supply lines 132. Panel drive video signals V(i) are video signals expanded in six phases from input video signal VIDEO by the phase expansion circuit 30. Therefore, each panel drive video signal V(i) contains pixel signals for every sixth pixel, and the frequency of the panel drive video signal V(i) is lower than the frequency of input video signal VIDEO. As a result, even if the operating speeds of the data-side drive circuit 130 and the scanning-side drive circuit 120, constituted of thin film transistors, are low, the data-side drive circuit 130 can reliably sample, with the sampling switches 134, pixel signal PD corresponding to each data signal line 112 from panel drive video signals V(1) to V(6) supplied to terminals VIN1 to VIN6 in accordance with sampling signals supplied from a shift register 136 for driving the sampling switches 134.
There is also a need to drive the liquid crystal panel by an alternating current signal. Therefore, the polarity of the liquid crystal drive video signal is always changed. Polarity inverting drive per dot is more effective in improving and stabilizing image qualities than polarity inverting drive per frame or polarity inverting drive per line.
Then, according to conventional art, as shown in FIG. 23, a polarity inversion circuit 40 is formed in a stage before the phase expansion circuit 30. In the polarity inversion circuit 40, a signal output circuit 42 forms two video signals inverse in polarity from the input video signal VIDEO and outputs these signals, and selectors 44a and 44b, formed of analog switches, change the polarity of the video signal supplied to each of the sample and hold circuits of the phase expansion circuit 30.
In the conventional image display apparatus, however, the phase expansion circuit 30 is provided with the circuits corresponding to the phases and these circuits may have different gains or offsets due to variations in their characteristics, changes with time of their component parts, or their mounted conditions, even though they have the same circuit configuration. Therefore, even if input video signal VIDEO has pixel signals PD uniform in brightness, there is a possibility of the pixel signals PD having different intensities with respect to the phases after phase expansion. The problem in such a case is that some of the pixels which are to be equal in brightness on the liquid crystal panel 110 will be displayed with different degrees of brightness. That is, if panel drive video signal V(i) having an abnormal intensity is supplied to some data signal line 112 in each group of six data signal lines 112, the corresponding difference in brightness appears as a vertical line on the liquid crystal panel 110.
Moreover, in the conventional image display apparatus, the selectors 44a and 44b handle video signals at high frequencies. However, the frequency of a video signal may be so high that they hardly follow up the signal. Therefore, when a display is made by using phase-expanded video signals, adaptation to video signals having certain high frequencies is impossible particularly if the display is a one-dot polarity inverting display.